The idea behind a cocaine vaccine is simple: trick the body into thinking the drug is a threatening disease. If a vaccinated person then takes the drug, their immune system will mop it up before it gets into the brain and its effects take hold. In theory, at least, the hit from the drug never arrives.

Most recreational drugs, including cocaine, heroin and ecstasy, consist of small molecules that can easily slip past the body's immune system and get into the brain. When cocaine enters the brain, it leads to a surge in the feelgood chemical dopamine.

To make a cocaine vaccine, Xenova Group scientists tag a derivative of cocaine to a harmless, but much larger cholera protein.

When the vaccine is injected, the immune system recognises the cholera protein as "foreign" and starts churning out antibodies. While some of these antibodies target the cholera protein, others will attack cocaine.

According to Xenova, after a course of injections spanning from four to six weeks, levels of the antibodies in the blood should be sufficient to block the effects of cocaine.

Once vaccinated, any cocaine that does get into the bloodstream will quickly be covered in antibodies that make the drug too large to pass across a natural filter called the blood-brain barrier that protects the brain.

"The result is, you don't get the high," said Xenova's director of clinical development, Simon Long.

Preventing a drug-induced high could help those who want to stay off the drug, though it will do nothing to ease withdrawal.

Some experts are sceptical that a cocaine vaccine could ever be effective enough. A line or two of cocaine contains around 200mg of drug, a large amount for the immune system to block. "Just working out the maths - you'll need so many of these antibodies to scavenge the cocaine as it flows to your brain, you're going to struggle to make it," said Professor John Henry, an expert in illicit drugs at St Mary's hospital in London.